Zonation on sandy beaches

The zonation of macrobenthic fauna on intertidal sandy beaches of open coasts is reviewed. Two major zonation schemes have been in use: a tripartite division of the shore, defined biologically, and a subdivision into four zones based on physical changes across the intertidal. Published accounts of zonation are evaluated in terms of these two schemes for South America, Central and North America, Africa, Australia, New Zealand and the Pacific, Europe and Asia. Most accounts are uncritical descriptions of zonation in three bands: supralittoral, eulittoral and lower-shore/subtidal. Recent studies suggest that zonation varies between beach types, fewer zones being present on reflective than dissipative beaches, because of the absence of fauna on the lower shore in the former. Many communities exhibit marked temporal variability, often coupled to seasons and reflected in changing zonation patterns. Rhythmic behaviour also varies and determines zonation. This is best developed in crustaceans, which display marked circatidal rhythms in the midshore, but also circadian rhythms, especially in the supralittoral and sublittoral, and sometimes weak circasemi-lunar rhythms. Intraspecific zonation by size has been recorded in many species (especially bivalves), juveniles being located above adults in some cases and below them in others. Competition and predation generally exert limited influence on zonation on open sandy shores. It is concluded that zonation on sandy beaches is dynamic and variable, usually being most distinct at the top of the shore and becoming less clear down shore. Three zones appear to be generally valid, the lower zones tending to disappear towards reflective beach states, but becoming more complex towards macrotidal dissipative conditions.